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AI'l'KNDIX                                    315

phate acts as a catalyst. The latter reagent has been used success-
fully in other oxidising processes, although the manner of its
action is not yet explained. The formation of phthalic acid from
naphthalene represents the initial stage in the manufacture of
artificial indigo from coal-tar. The subsequent processes consist
in converting the acid into the anhydride by sublimation., the
anhydride: into phthalimide by the action of ammonia yas, and the
phthalimide into anthranilic acid by the action of sodium hypo-
bromite (Hofinann's reaction, see p. 80).

co-.                  /CON m:r

Nil -> CV.1I/                ->

Co                        X'OOIl


The anthranilic acid is then converted into indigo by
combining it with ehloracetie acid and fusing the product with
caustic alkali, which gives indoxyl and finally indigo by oxida-

.N'll.,                                            /NIICIUCOOII

i ciciL.cooii - tyi/

"             '    ' coon


PRKPA RATIONS 105 and 106.
Naphthalenesulphonate of Sodium. /3-Naphthol.-
The formation of the sulphonic acid of naphthalene and the
corresponding phenol by fusion with caustic soda is analogous
to that of ben/.ene sulphonic acid and phenol (see Prep. 74,
p. 177,111x176, p. 179. It should be noted that naphthalene
forms two series of mono-derivatives distinguished as a and ft
compounds. By the action of sulphuric acid on naphthalene,
both (i and # sulphonic acids are formed. At a lower tempera-
ture (100 ) the product consists mainly of the a compound ; at a